buildman: Provide a hint on how to debug thread crashes

If a thread crashes it is helpful to try the operation again with
threading disabled. Add a hint about that.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/builderthread.py

@@ -557,6 +557,6 @@ class BuilderThread(threading.Thread):
             try:
                 self.RunJob(job)
             except Exception as e:
-                print('Thread exception:', e)
+                print('Thread exception (use -T0 to run without threads):', e)
                 self.builder.thread_exceptions.append(e)
             self.builder.queue.task_done()

tools/buildman/func_test.py

@@ -623,4 +623,6 @@ class TestFunctional(unittest.TestCase):
         with test_util.capture_sys_output() as (stdout, stderr):
             self.assertEqual(102, self._RunControl('-o', self._output_dir,
                                                    test_thread_exceptions=True))
-        self.assertIn('Thread exception: test exception', stdout.getvalue())
+        self.assertIn(
+            'Thread exception (use -T0 to run without threads): test exception',
+            stdout.getvalue())